1. Field of the Invention
The invention is directed to imaging systems, and more particularly to radiographic X-ray imaging systems, for medical, industrial, and other applications.
2. Description of Related Art
Radiographic X-ray imaging systems for medical, industrial and other applications typically use a point-source X-ray tube in which energetic electrons impinge upon a solid metal target thereby producing a cone-beam of X-ray light emanating from the focal spot. The spectrum of X-rays emitted from such tubes is poly-energetic, having line emission characteristic of the anode material used in the tube (commonly tungsten, or in the case of mammography, molybdenum or rhodium) superimposed on a broad continuum of Bremsstrahlung radiation extending to a high-energy cutoff determined by the applied voltage. For many imaging tasks, however, increased image contrast—and lower patient dose, in the case of medical applications—can be achieved using mono-energetic radiation.
One method for producing (nearly) mono-energetic radiation from electron-impact X-ray tubes utilizes multilayer X-ray mirrors to reflect and filter the X-ray light before it reaches the tissue or sample under study. [See, for example, ‘X-ray monochromator for divergent beam radiography using conventional and laser produced X-ray sources’, H. W. Schnopper, S. Romaine, and A. Krol, Proc. SPIE, 4502, 24, (2001)]. The X-ray mirrors include flat substrates coated with X-ray-reflective multilayer coatings that reflect X-rays only over a narrow energy band. The multilayer X-ray mirrors are positioned between the X-ray tube focal spot and the sample or patient. Because the mirrors only work at shallow grazing incidence angles, a single mirror will only yield a thin fan-beam of mono-energetic X-ray light. Thus, to produce mono-energetic light over a large field at the image plane, one of two approaches can be used. In the first approach, a single mirror is scanned over a wide angular range during the X-ray exposure. In the second approach, an array of stacked mirrors are used, constructed from a number of thin mirrors and spacers that are stacked together with high precision in a wedge shape: while each individual mirror will produce a narrow fan beam, the array of mirrors will collectively produce an array of co-aligned fan beams. In the second approach using a mirror stack, however, the illumination pattern will also include dark strips corresponding to the regions where the X-ray light is blocked by the edges of the mirrors. To compensate for the dark strips, the mirror stack can be scanned during exposure, similar to the way in which a single mirror is scanned in the first approach (albeit over a much smaller angular range), so that the bright and dark strips are averaged together to produce uniform illumination.
In any case, the requirements on positioning the mirrors relative to the focal spot are stringent: in particular, the angular position of each mirror must be such that the incidence angle of X-rays is controlled to a fraction of a degree. As an example, in the specific case of multilayer X-ray mirrors designed for mammography systems operating near 20 keV, approximately, typical grazing incidence angles are in the range of 0.3-0.7 degrees, while the angular acceptance angle of the narrow-band multilayer coating can be as small as 0.02 degrees; therefore the mirror must be positioned so that the error in graze angle is perhaps half of the acceptance angle, i.e., 0.01 degrees, or less. For other types of X-ray imaging systems utilizing higher-energy X-rays, the graze angles and acceptance angles are even smaller, and thus the requirements on alignment are even more stringent than for mammography.
For medical applications in particular, point-source X-ray systems generally incorporate a visible-light alignment system for patient registration, i.e., to ensure, by visual inspection of the optical illumination pattern, that the X-ray beam will illuminate the desired portion of the tissue under study. The visible-light alignment system is arranged to mimic the X-ray beam by implementing a small incandescent light bulb positioned at a virtual focal spot location, with the light emitted from the bulb reflecting off a 45-degree mirror (having low X-ray attenuation) positioned in the X-ray beam. [See, for example, ‘The Essential Physics of Medical Imaging, 2nd Edition’, J. T. Bushberg, J. A. Seibert, E. M. Leidholdt, Jr., and J. M. Boone, Lippincott Williams & Wilkins publishers, Philadelphia, 2002, FIGS. 5-18, pg. 115.]
A visible light alignment system still can be used for patient registration when X-ray mirrors are implemented in a radiographic X-ray imaging system as outlined above: provided the visible-light is sufficiently co-aligned with the X-ray light, the visible light will reflect from the mirrors and accurately illuminate the image field, just as it does in conventional systems. The same visible light alignment system could also be used to align the mirrors themselves, in principle, which is an otherwise difficult task, again provided that the visible/X-ray misalignment is sufficiently small. U.S. governmental regulations relating to radiographic X-ray imaging systems (21 CFR §1020.31) require that the visible light field and X-ray field at the image plane be co-aligned such that the sum of the misalignments, along either the length or width of the field, is less than 2% of the distance from the X-ray focal spot to the image plane. While the misalignments permitted under governmental regulations are adequate for patient registration, such misalignments are completely inadequate for use with the X-ray mirrors designed to produce mono-energetic radiation that operate a relatively shallow grazing incidence angles: i.e., when X-ray mirrors are placed in the X-ray beam, the typical misalignments between the visible and X-ray beams in a conventional alignment system will make it difficult or impossible to use the visible beam to align the X-ray mirrors. Additionally, conventional visible-light alignment systems do not provide sufficiently precise adjustments of the position of the visible light source relative to the virtual focal point, and furthermore, the size of the light-emitting region itself is large relative to the X-ray focal spot size. Thus, conventional visible light systems are generally inadequate for use when X-ray mirrors are implemented.
In order to utilize a visible-light alignment system that is similar in concept to the systems currently in use, either for patient registration or to align X-ray mirrors implemented for mono-energetic radiation, an optical system with significantly increased precision is required. Furthermore, for precise optical alignment of the X-ray mirrors, the visible-light optical system must use a visible light point source having a focal spot whose size is equal to or smaller than the X-ray tube focal spot size. Finally, also required is an apparatus and a methodology for precisely co-aligning the visible and X-ray light cone-beams.